Nearly all conventional valves such as gate valves, ball seat valves and plug operated valves employ driven threaded members. Since these members effect the large mechanical advantage necessary to move the valve element against what is often times a considerable hydrostatic pressure, they are generally characterized by their relative slowness in operation. That is, from full opened to full closed, the control element -- usually a threaded stud or worm gear -- requires significant movement or rotation that tends to make these valves comparatively slow acting.
Slow acting valves are ill suited for emergency control. Hence, it is not uncommon to find some pipe systems with two valve schemes. One scheme is used for normal control operation and the other scheme is for emergency control. The latter typically is of the "go" or "no go" variety with no intermediate positioning. This last mentioned emergency scheme might be an explosively operated gate valve designed to nearly instantaneously imposition a blade across the pipe and block fluid flow. Of course, other motive means are available for emergency operation such as solenoids and pressure operated devices but being for emergency control most are not capable of controlling or regulating fluid flow in the conventional sense. This birfurcation control has, in certain critical pipe designs, lead to the requiring of both regimes with its attendant redundancy and cost. For example, in an oil pipeline transmission system such as the Alaska pipeline the nature thereof mandates many hundreds of yards if not miles between pumping stations. For the most part the pipeline, which is forty-eight inches in diameter, remains unattended. A break in this pipeline could do serious environmental damage if not quickly controlled. Thus it is desirable to provide an effective, reliable and low cost valve arrangement that could be closely spaced along the line and that has nearly instantaneous shut off capabilities. Yet conventional valving arrangements appear other than adapted for this purpose.
Then, too, in drilling for oil an effective and reliable valve arrangement is required that affords a nearly instantaneous shutting down or closing of the drill pipe or casing. Such a valve arrangement is needed because often times when the well is "struck" the fluid blows out under tremendous pressure. As the gas or oil expands through the well head into the ambient atmosphere, it atomizes increasing the fluid's surface area. In the oxygen rich ambient atmosphere the expanded gas is prone to ignite and often does. Oil and well fires are notorious for their intensity, they are a hazard to life and property and the blow out, whether ignited or not, pollutes the environment. Hence it is highly desirable to effect a nearly instantaneous shutting down of such a "wild" gas or oil flow to reduce and nearly eliminate the fire hazard.
Yet conventional valving apparatus is other than adapted for reliable and rapid flow stoppage because of the severity of the drilling environment. Obviously most, if not all of the drill piping is below ground thus the emergency valve must operate in a dirt and mud soaked environment. Irrespective of the seals employed, abrasive particulates find their way into the critical valve control elements often times leading to faulty operation if not total immobilization. Then again, the vibrations and stresses placed on both piping and valves during normal drilling effects a rapid fatigue of their structural elements, not to mention the shock loading and vibrational intensity reached when the fluid from a high pressure gas or oil pocket "blows out" through the drill pipe and well head. These unique, albeit severe loading factors have made conventional shut-off valves unreliable at best.